City Council Speaker Christine Quinn's journey from mayoral favorite to also-ran in the Democratic primary Tuesday might have been as inevitable as it was surprising to her core supporters, some of those backers and other political observers said.

Ms. Quinn's strengths—high name recognition, prodigious fundraising and familiarity with the workings of city government—seemed of little help in a Democratic primary that in the end turned most on the promise by her Democratic rivals of changing the status quo.

She may have been most vulnerable because of her close relationship with Mayor Michael Bloomberg, those experts said. By working with the three-term mayor to build the record she would eventually run on, it became difficult for Ms. Quinn to escape the shadow of Mr. Bloomberg when many voters seemed to tire of him, they said.

"Mayor Bloomberg was the hand that Chris was dealt," said former state Sen. Thomas Duane, Ms. Quinn's former boss and mentor, the day after she finished in third place, well behind Public Advocate Bill de Blasio and former Comptroller Bill Thompson. Months earlier, she had towered over both in public polls of the Democratic field.

Her position might never have been as strong as it appeared in polls, to some fellow candidates, and in the news media, some said. "With 20/20 hindsight, she may well have been in an impossible situation," said Kenneth Sherrill, professor emeritus of political science at the City University of New York Graduate Center and Hunter College.

"Doing the job well as speaker is not something that's terribly conducive to winning primaries," Mr. Sherrill said. "It's nothing personal. It comes with the turf."

To Peter Vallone Sr., another former council speaker who tried and failed to make the leap to mayor, her position looked a lot like that of former Republican Sen. Bob Dole, who failed in his 1996 contest against Democrat Bill Clinton for president.

"What happens is as soon as a legislator runs, particularly one who's in charge of a legislature, you have a record of doing things," Mr. Vallone said.

Ms. Quinn's campaign highlighted some instances of independence from Mr. Bloomberg, such as her backing of a bill to create an inspector general for the police department or a paid sick-leave mandate for city business. But she didn't shrink from working closely with the administration, or expanding on some of its previous work during her campaign.

Ms. Quinn didn't emphatically break with Mr. Bloomberg, even when doing so might have curried favor with some factions of the Democratic base, election watchers said. While expressing concern about the controversial police tactic of stop-and-frisk, Ms. Quinn was the only major Democratic candidate who said she would keep Raymond Kelly as police commissioner. And she was hammered by her rivals for leading the override of the city term-limits law to give Mr. Bloomberg his third term.

Exit polls showed that there was "obviously some Bloomberg fatigue" among primary voters, Quinn spokesman Mike Morey said Wednesday, and the campaign endured a long drubbing from rivals when the speaker was the front-runner.

But it wasn't clear until late in the race, he said, that Democratic voters wanted "a 180" reversal from Mr. Bloomberg's time in office. "She's never been afraid to oppose the mayor. She's done it on multiple occasions in very high-profile situations," Mr. Morey said.

A spokeswoman for Mr. Bloomberg declined to comment.

Mr. Vallone and some other Democrats interviewed Thursday predicted a return to politics in some form for Ms. Quinn.

Among them is Gifford Miller, another former speaker and unsuccessful mayoral candidate.

"I think Chris Quinn is an extraordinarily talented public servant and she has a very bright future, and I'd be very surprised if we've seen the last of her," said Mr. Miller, who is now a real-estate developer.

Ms. Quinn's tenure as speaker has three months remaining. That includes Thursday morning, when she is slated to preside, once again, over a meeting of the City Council.

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